I am aware of certain prior art patents that describe flow control valves for pumps that are speed sensitive, one being U.S. Pat. No. 4,244,389. That patent comprises a flow control valve having a variable orifice, the effective area of the orifice depending upon the position of the spool valve as seen in FIGS. 1 and 2 of the drawings for that patent. The valve moves, however, in accordance with pump speed or engine speed to vary the flow of fluid to the steering gear. The valve has no road speed sensitive function for eliminating the power assist at a selected design speed as in the present disclosure.
U.S. Pat. No. 3,314,495 shows a well known flow control valve having a so-called drooper pin, best seen in FIG. 6 of that patent. The drooper pin is located in a flow control orifice that communicates with the fluid supply passage for a power steering gear. As the valve spool moves, the metering pin also moves; and its tapered area registers with the orifice to provide a variable effective orifice restriction. As the pump speed changes, the pressure of the fluid in the spring chamber for spring 34C for the construction of FIG. 6 of that patent is reduced by the venturi effect of the increased flow upon an increase in pump speed.
Power steering gear valves, as distinct from power steering pump valves, are shown in U.S. Pats. Nos. 3,690,400 and 3,692,137. These control the magnitude of the pressure in a reaction chamber for a power steering gear. They sense a speed signal, but they are effective only to influence the magnitude of the torque reaction during steering maneuvers and are not effective to reduce pump horsepower loss of the associated power steering pump and to eliminate the power assist at high speeds.
Another power steering system valve shown in U.S. Pat. No. 4,085,657 comprises a differential pressure operated valve that acts in a power steering gear fluid circuit to alter the fluid flow to the steering gear thus changing as desired the characteristic relationship of pressure to steering effort.